Hollow core photonic crystal fibers (PCF) have been known for many years and in some applications are very attractive to use compared to solid core PCF due to their very low propagation loss, low nonlinear effects and high damage threshold.
A hollow-core fiber is an optical fiber which guides light essentially within a hollow region, so that only a minor portion of the optical power propagates in solid fiber material surrounding the core. The hollow region may be filled with air or any other gas or it may be evacuated to have a very low gas pressure.
Over the years several types of hollow core PCFs have been developed, such as hollow-core photonic bandgap fibers e.g. as described in U.S. Pat. No. 6,892,018, Kagome design type fibers e.g. as described in U.S. Pat. No. 8,306,379.
The kagome type fiber guides light by means of an anti-resonant effect and allows for a substantially broader spectral transmission than that achieved in photonic bandgap fibers.
In recent years such hollow core anti-resonant fibers (ARFs) have been studied intensively in particular with the aim of further improving the fiber with respect to low loss and broad spectral transmission.
In an attempt to reduce the attenuation Kolyadin et al, Optics Express, Vol 21, pages 9514 to 9519, 2013 suggested a hollow core fiber with a hollow core surrounded by eight identical capillaries. It was concluded that to reduce loss, a larger core size is preferred and by increasing the number of capillaries in the cladding a reduced loss could be achieved.
Another drawback of prior art hollow core anti-resonant fibers is that they are not purely single mode waveguides because they also support some higher-order modes (HOMs). Since these HOMs often have a relatively low loss, it is very difficult to launch a pure LP01 mode without HOM contamination and further HOMs can be excited by bending or external stress.
In an attempt to solve this problem and to achieving the highest possible suppression of HOMs while maintaining a reasonably low loss for the LP01 mode, a modified fiber design was suggest in “Broad-band robustly single-mode hollow-core PCF by resonant filtering of higher order modes” by Günedi et al., Cornell university library arXiv:1508.06747 [physics.optics], 27 Aug. 2015. The ARR fiber described in this article comprises a central hollow core (inner diameter D) surrounded by six evenly spaced and non-touching capillaries (in the following also called hollow tubes) with a wall thickness t and an inner diameter d, supported within a thick-walled supporting capillary. The non-touching glass elements had modes which were tailored to ensure resonant phase-matched coupling to higher-order core modes, causing them to leak at a very high rate into the supporting solid glass sheath. It was found that extremely high suppression of HOMs could be achieved over broad wavelength bands where the fiber guides with a low loss, provided the ratio d/D≈0.68 and the walls of the tubes walls T were thin enough i.e. t/D=0.01.
WO15185761 describes a similar anti-resonant hollow-core fiber comprising a first tubular, cladding element which defines an internal cladding surface, a plurality of second tubular elements which are attached to the cladding surface and together define a core with an effective radius, the second tubular elements being arranged in spaced relation and adjacent one of the second tubular elements having a spacing there between, and a plurality of third tubular elements, each nested within a respective one of the second tubular elements. It was concluded that the optimal number of tubes for suppressing HOMs was six.